杏吧原创

Giant superheated bubbles of gas are drifting towards Earth and popping as they encounter our planet's magnetic field, new findings reveal. Researchers suspect the bubbles may actually feed the "bow shock" that is created where the solar wind rams into our planet's magnetic field.The charged particles of the solar wind generate the bow shock 鈥� similar to the bow wave in front of a ship 鈥� when they ram into and compress Earth's magnetic field. The bow shock then slows down and deflects the bulk of the incoming solar wind around the Earth's magnetic field. The magnetic field then protects the planet from most of the dangerous particles that could harm people and disrupt satellites and power grids."But fundamentally, we don't know how charged particles in space can create a shock," says George Parks of the University of California, Berkeley, US. The evidence is mounting, however, that a "zoo" of bubble-like phenomena might play a role in building up the bow shock. Previous observations by individual spacecraft had revealed hot bubbles of gas beyond the bow shock that appeared for a minute or more before popping. But these bubbles, called hot flow anomalies, were rare 鈥� previous missions detected only 30 over a period of five years.Now, Parks and an international team of collaborators have taken advantage of a fleet of four European Space Agency spacecraft called Cluster and one Chinese spacecraft, Double Star, to discover a new type of bubble that appears to be very common.Hot poppingThe spacecraft orbit Earth at various distances beyond the bow shock, and each observed the same bubbles from different viewpoints as they drifted towards the planet and popped. The bubbles formed when the density of the tenuous gas present in space dropped by a factor of 10, and the temperature of the gas left rose from 100,000掳C to 10 million掳C.These so-called "density holes" grew to span more than 1000 kilometres but were short-lived, surviving for an average of just 16 seconds. And they were much more common than the hot flow anomalies seen previously&colon; 150 density holes were observed in less than a week of observations.Inside the density holes, solar wind particles stopped moving towards Earth and were pushed to the sides of the bubbles. "The solar wind is somehow being deviated very, very dramatically," Parks told New 杏吧原创.Whatever that process is, it is similar to what happens on a larger scale when the solar wind hits the bow shock, Parks says. That suggests that these density holes could collectively form the bow shock&colon; "We could be seeing the elementary structure of the large-scale bow shock," he says.Model impossibleThe underlying physics of the density holes is still not understood because today's computers are not powerful enough to model the behaviour of every particle in a hot gas, Parks cautions. "Currently, there is no theory available to describe the density holes or some other bubble-like features that have been seen in the past," he says.But the new discovery should help physicists understand how shock waves form and behave in space. "These shocks are seen throughout the universe," says Parks, adding that they form around supernova explosions. "They're important for understanding the acceleration of particles."Journal reference&colon; Physics of Plasmas (vol 13, p 050701)